We propose to develop a revolutionary tool for simultaneous intracellular recording from hundreds of single neurons in the freely-moving animal. In order to link neural activity and behavior it is essential to record the electrical activity of many individual neurons simultaneously for long periods of time. Current methods allow recording of multiple neurons only at the spike level, losing the crucial details that are available in subthreshold synaptic activity. Intracellular recordings in awake freely moving animals has been achieved only a handful of times, one neuron at a time and for very brief periods. Here we propose a novel approach for solving this problem by developing in vivo multielectrode array (MEA) platforms that implement the IN-CELL recording approach - using extracellular electrodes to provide intracellular signals ? which we have developed and demonstrated using in vitro systems. The groundbreaking bioengineering process underlying the IN-CELL recording approach consists of luring neurons to tightly engulf micrometric sized gold mushroom-shaped microelectrode (gME) functionalized by bioactive materials. This unique design enables the formation of a tight seal resistance by the cleft between the neuron's plasma membrane and the gME ?neck?, together with an increased conductance of the junctional membrane that faces the gME. The configuration enables effective intracellular recordings and stimulation and, in in vitro systems, is stable over long periods of time (weeks). Our unique design, experience and know-how, along with the most advanced concepts and technologies developed by the international bioengineering community for flexible in vivo MEA platforms, will be used to develop, manufacture, test and apply in vivo MEAs carrying gMEs that match the recording and stimulation qualities of intracellular microelectrodes and will be used for acute or chronic recordings of the entire electrophysiological repertoire. The technology will be scalable and allow recordings from hundreds of individual neurons in multiple brain regions.